1. Field of the Invention
This invention pertains generally to aquatic sensors, and more particularly to a sonification method and apparatus for preventing biofouling of dissolved oxygen and other aquatic sensors.
2. Description of the Background Art
Dissolved oxygen and other electrochemical sensors are commonly used to detect the presence of substances such as dissolved gases and ions in water. Typically, these sensors have one or more electrodes immersed in an electrolyte solution and are covered by a membrane that is permeable to a substance of interest. To measure the concentration of a substance of interest, the sensor is submerged in the solution to be analyzed and, as the substance of interest diffuses through the membrane, it causes a chemical reaction between the electrolyte solution and the electrode. This reaction generates an electrical signal that is detected by a meter, and is indicative of the concentration or activity of the substance of interest. For dissolved oxygen (DO) sensors, the electrolyte is usually replaced on the order of every two to four weeks. However, if the sensor is submerged in a biologically active water (e.g., lakes, rivers, wastewater treatment plants), biofouling organisms can start adhering to the membrane surface in a matter of hours. Over a time period of much less than two weeks, the biofilm that develops on the membrane surface will adversely affect the sensor accuracy.
In response to this problem, various approaches to eliminating or controlling biofouling on the surface of aquatic sensors have been developed. These approaches typically fall into the following four categories:
(a) Electrochemical cleaning. In this approach a timed reaction releases biostatic or biocidal chemicals in the vicinity of the membrane surface. PA1 (b) Mechanical cleaning. This approach is usually characterized by vigorous vibration or agitation of the sensor and/or contact abrasion by suspended particles or a wiping device. PA1 (c) Hydraulic cleaning. Here, timed jets of water or chemical solutions bathe the sensor surface, washing and dislodging any attached fouling. PA1 (d) Software compensation. Here, a quantitative model of sensor attenuation due to the biofouling layer is obtained. The probe signal is then adjusted by this amount to compensate for the biofouling.
In practice, a person designing cleaning systems for sensors used in sensitive aquatic systems must avoid the use of harsh chemical treatments, harmful biocides, and the alteration of temperature, pH, or the concentration of the substance of interest in the water being tested. Current mechanical and hydraulic cleaning systems add a level of unnecessary complexity to the sensor and are fraught with problems of their own, such as fouling of the nozzles and the need for additional pumping systems and motorized parts. Furthermore, accurate software compensation is extremely problematic due to the difficulty in developing an accurate and robust model of biofilm growth.